Ligation of the T cell antigen receptor (TcR) does not result in a single series of events, but rather can lead to a wide variety of different outcomes including proliferation, cytotoxicity, apoptosis, distinct patterns of lymphokine secretion, and anergy. The precise mechanisms responsible for regulating these different outcomes remains unclear. One molecule important in regulating T cell activation is CD45, a family of high molecular weight transmembrane glycoproteins consisting of a cytoplasmic tail containing two domains with protein tyrosine phosphatase activity, and a variable external domain encoded in three exons which are differentially expressed due to post-transcriptional alternative RNA splicing. This gives rise to a variety of different isoforms whose expression is regulated during T cell development and following T cell activation, and is correlated with different T cell effector functions. The cytoplasmic tyrosine phosphatase domains of CD45 appear to be critical for T cell activation since mutant T cells which lack CD45 are refractory to T cell receptor signaling, and since signaling events are restored following transfection with CD45, or with heterochimeric molecules which lack the external domains but retain the tyrosine phosphatase domains. These data indicate that the common cytoplasmic domain of CD45 is directly involved in the regulation of second messenger generation after signaling through the TcR, however, the role of the variable isoforms of CD45 in regulating T cell activation is uncertain. There are two main hypotheses for this role (not mutually exclusive) which are to be explored in this proposal. The first hypothesis is that the different isoforms of CD45 may regulate signaling by preferentially associating with other cell surface molecules on the same cell (such as CD4). A second hypothesis is that the different isoforms may be involved in regulating adhesion during cell-cell interactions and perhaps influence T cell interactions with different antigen presenting cell populations. In order to test these hypotheses, a series of T cell transfectants will be made which express different CD45 isoforms on cells with a defined T cell receptor. The effect of individual CD45 isoforms on antigen-specific T cell activation will be determined. Transfectants will also be constructed which express different extracellular isoforms of CD45 linked to the membrane by a glycolsyl phosphatidyl inositol linked anchor. These cells will be used to determine the role of the extracellular domains in the absence of the cytoplasmic phosphatase domains, and to produce soluble CD45 isoforms which will be used as a probe to detect isoform specific CD45 ligands, and evaluated for the ability to modulate antigen-specific T cell activation. These studies on the regulation of T cell activation have broad applicability to fields of cancer pathogenesis, autoimmunity, and infectious disease.